topN query

select  * from emp where rownum=1   or   rownum=2;
select * from   (select * from emp order by    sal desc) where rownum<5;

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Paging query

1. 
   select * from (select rownum no,e.* from(select * from emp order by sal desc) e where rownum<=5) where no>=3;
2. 
   select * from (select rownum no,e.* from(select * from emp order by sal desc)e) where no>3 and no<=5;

exist

select * from t1 where exists (select null from t2 where y=x);

for x in (select * from t1)
loop
    if(exist(select null from t2 where y=x.x))

    then
    output the record
     end if;
end loop;

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select 1 from dual where null in (0,1,2,null);

Multiline subquery

select * from emp   where sal>any(select avg(sal) from emp group by deptno);

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select * from emp   where sal>all(select avg(sal) from emp group by deptno);

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select * from emp   where  job in (select job from emp where ename='MARTIN' or ename='SMITH');

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Random return of 6 records

select * from (select ename,job from emp order by dbms_random.value())      where rownum<=6;

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Process null sort last(first)

select * from emp order by comm desc nulls last;

Query skip even rows in table

select ename from (select row_number() over (order by ename) rn,ename
from emp) x where mod(rn,2)=1;

Find employee information and the employee with the highest and lowest salary

select ename,sal,max(sal) over(),min(sal) over() from emp;

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Continuous summation

select ename,sal,sum(sal) over(),sum(sal) over(order by ename) from emp;

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Continuous sum of sub sectors

select deptno,sal,sum(sal) over (partition by deptno order by ename) as s from emp;

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Get data from previous or next row

select ename,sal,lead(sal) over (order by sal) aaa,lag(sal) over(order by sal) bbb from emp;

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select [month] ,sum([income]),lead(sum ([income])) over(order by sum([income]))   from [table] group by [month];

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Group by substring

select to_char(hiredate,'yyyy'),avg(sal) from emp group by to_char(hiredate,'yyyy');

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Determine the number of days in a year

select add_months(trunc(sysdate,'y'),12)-trunc(sysdate,'y')  as Days from dual;

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1.

select deptno,ename,sal from emp e1 where (select count(1) from emp e2 where e2.deptno=e1.deptno and e2.ename!=e1.ename and e2.sal>e1.sal)<2 order by deptno, sal desc;

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2.

select * from (select deptno ,ename,sal,row_number() over (partition by deptno order by sal desc) rn  from emp)  where rn<3;

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data dictionary

Query all tables under a user

select table_name from all_tables where owner='SCOTT';

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select * from all_tab_columns where table_name='TEMP';

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select *from  sys.all_ind_columns where table_name='TEMP';
select * from sys.all_ind_columns where upper(table_name)='CAREUSERHAM';

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select  * from all_constraints where table_name='TEMP';

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select table_name,comments from dictionary where table_name like '%TABLE%';

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